This invention relates to waveform demodulation, and more particularly to methods of and apparatus for receiving and demodulating digitally modulated signals and analog amplitude modulated signals within the same frequency channel assignment.
Broadcast and reception of digitally-encoded audio signals is expected to provide improved audio fidelity. Several approaches have been suggested. Out-of-band techniques provide broadcast of digital radio signals in a specially designated frequency band. In-band techniques provide broadcast within substantially vacant slots between adjacent channels in the existing broadcast band (interstitial approach) or in under-utilized portions within the same frequency channel allocations currently used by commercial broadcasters (in-band on-channel or IBOC approach). The in-band on-channel approach may be implemented without the need for additional frequency coordination and with relatively minor changes to existing transmitting equipment. It is also a requirement that any digital audio broadcasting (DAB) technique must not degrade analog signal reception by conventional analog receivers.
In-band approaches to digital audio broadcasting have thus far only been proposed in the FM band (88 MHz to 108 MHz), since the bandwidth of AM channels is relatively narrow as compared to the FM band allocation. However, high fidelity digital audio broadcasting in the AM band (530 kHz to 1700 kHz) would provide AM broadcasting stations with a means to compete with high-quality, portable audio sources such as cassette tapes and compact disc players. It would therefore be desirable to promote an in-band on-channel (IBOC) approach in the AM broadcasting band to provide enhanced fidelity through digital signalling without affecting reception by existing analog AM receivers.
An AM compatible digital broadcast waveform which satisfies the requirement of substantial orthogonality between a conventional analog AM signal and a digitally modulated signal set has been developed. The waveform is described in U.S. Pat. application Ser. No. 08/206,368 filed Mar. 7, 1994, entitled METHOD AND APPARATUS FOR AM COMPATIBLE DIGITAL BROADCASTING. The waveform spectrum consists of in-phase and quadrature components. An in-phase radio frequency (RF) carrier is modulated by an analog audio signal and the in-phase component of a digital signal. The quadrature-phase RF carrier is modulated by the quadrature component of the digital signal. The digital signal has an orthogonal frequency division multiplexed (OFDM) format. The in-phase signal consists of the conventional analog AM signal and selected digital carriers. The in-phase digital carriers are placed outside of the spectral region occupied by the analog AM signal. The quadrature-phase carriers are situated both within and outside the spectral region occupied by the analog AM signal (although not at the center frequency occupied by the unmodulated analog carrier). The quadrature digital carriers situated within the same spectral region as the analog AM signal are called complementary carriers. The above described arrangement is further described in U.S. Pat. application Ser. No. 08/368,061 filed Jan. 3, 1995, entitled METHOD AND APPARATUS FOR IMPROVING AM COMPATIBLE DIGITAL BROADCAST ANALOG FIDELITY.
The context of the present invention is a need to demodulate the composite waveform with minimal crosstalk. The modulated composite waveform is produced by a modulation method in which an analog amplitude modulated (AM) signal and a digital signal which may be a representation of the analog audio signal (or it may be any other digital signal) are encoded together and transmitted simultaneously in the same frequency channel. This approach places some of the digital carriers in quadrature with the analog AM, thereby enabling the AM DAB data to be extracted and decoded with high fidelity and without crosstalk, assuming the receiver is capable of proper signal separation.
A receiver (which is not necessarily prior art) has been considered which converts the signal to baseband using conventional I and Q mixers, with the I channel signal being passed through a digital high pass filter to separate the digital signal from the analog signal, as hereinafter explained, a problem has been discovered related to crosstalk between the analog signal and the digital signal. In the receiver under consideration, the analog signal interferes with the demodulation of the complementary carriers if the demodulation of the I and Q component samples is carried out in a single common FFT processor. What is needed is a demodulation technique which minimizes the undesired crosstalk between the analog signal and the digital signals.